Concrete Core Research Articles

Cross-sectional shape effect of stay-in-place formwork column on axial compressive behaviour

Experimental and numerical studies on the behavior of composite columns with prefabricated self-compacting mortar (SCM) stay-in-place (SIP) formwork and post-cast normal-strength core concrete were presented.The axial compression of twenty-eight reinforcement concrete (RC) composite columns having square or circular cross-section shapes, with formwork thicknesses of 10 mm or 15 mm, and confined with one or two layers of Carbon Fibre Reinforced Polymer (CFRP (, were tested. A finite element (FE) model was generated to simulate the stress–strain relationship, and failure mode of the column using ABAQUS software. The results show that square cross-section-shaped columns exhibited lower axial load and displacement capacities than their counterparts of equivalent circular cross-section-shaped columns. This behavior maintained consistent regardless of the use of the SCM-SIP formworks or the wrapping with CFRP fabrics. Among the various confinement systems compared in this study, the efficiency of the CFRP was the greatest. Good bonding with no peeling failure was observed between SCM-SIP formwork and core concrete, indicating the good integrity of the composite columns. Consistency between the FE model and experimental results was observed for both cross-section shapes, indicating the precise and plausible characteristics of the model to sensibly estimate the stress-strain behaviors.

Structural performance of circular CFST columns with circumferential gap imperfections

The present study investigated the influence of circumferential uniform gap imperfections (≤ 1.0%) on the structural behavior of circular concrete-filled steel tube (CFST) columns under axial compression. The study involved testing 20 CFST columns with circumferential uniform gaps with varying gap ratios (0.2%–1.0%), diameter-to-thickness ratios (18–43), and concrete strength (68.5 MPa and 114 MPa). The presence of uniform gaps (0.2%–1.0%) reduced the confinement effect of steel tube on core concrete, and peak load capacity decreased by 9.0%–24.0%. The normalized concrete strength in high-strength concrete specimens with gap ratios ranged between 0.89 and 0.99, showing no interaction between steel tube and core concrete due to the less deformable nature of high-strength concrete. Core concrete exhibited a combination of vertical splitting and diagonal cracks distributed across the height of specimens with an increase in gap ratio, and local steel tube bulging was dispersed around the edge regions. A theoretical procedure was developed to assess the impact of uniform gaps on the stress state of steel tube and core concrete by determining their respective load contributions. The suggested approach reasonably estimated the gap ratio's nominal influence on the behavior of steel tubes and its considerable influence on the behavior of core concrete. Considering the experimental limitations, gap ratios were broadened through finite element modelling to investigate the gap ratio effect further. After examining the confinement mechanism of CFST columns and finite element results, an analytical formulation to predict the peak load capacity of CFST columns with circumferential uniform gaps was proposed and verified.

Experimental investigations into stocky composite columns of concrete-filled circular S690 steel tubes under compression

Composite columns of concrete-filled steel tubes (CFSTs) are very efficient in providing large structural resistances and rigidities. However, high strength S690 steel tubes are not covered in current design codes. Therefore, a series of experimental investigations into stocky composite columns with circular CFSTs under compression were conducted. A total of 21 stocky composite columns with different tube diameters and thicknesses made of Grades S355, S460 and S690 steel and Grade C60/75 concrete were tested under different loading conditions. Moreover, extensive measurements on the surface strains of the steel tubes were made during tests. Data analyses on the test results reveal that all composite columns exhibit enhanced section resistances and ductility at large deformations, indicating that the use of S690 steel and C60/75 concrete in these composite columns are highly efficient. A simple method for determining confining pressures provided by the steel tubes to the concrete cores in these composite columns is proposed, which involves a systematic analysis on the measured data and the use of von Mises yielding criterion without any complicated plasticity formulation. Based on the proposed method, the resistance contributions of both the steel tubes and the concrete cores are successfully quantified. Comparison between the measured and the predicted resistances using EN 1994–1-1 of these composite columns reveals that while the section resistances of these composite columns are safely predicted, the resistances of the steel tubes are underpredicted while those of the concrete cores are overpredicted correspondingly. The findings highlight the need to improve the predictability of the current design rules, and to give increased resistances of the steel tubes when high strength S690 steel tubes are used.

Analysis of fly ash-treated SDCM pile behavior through laboratory model tests

Fly ash, an industrial byproduct, has resulted in significant environmental pollution and poses a threat to human health due to its low recovery and utilization rate. Stiffened deep cement mixing (SDCM) piles, leveraging both the high lateral friction resistance of cement mixing pile socket and the high strength of concrete core pile, are widely employed in practice of soft ground improvement. However, to avoid damage to cement mixing pile socket, a higher amount of cement is often required, leading to not only elevated project costs but also conflicting with low-carbon environmental objectives. In tackling this concern, the introduction of fly ash as a partial substitute for cement in cement mixing pile socket offers a solution. This study delves into the vertical bearing mechanism of fly ash-treated SDCM piles through laboratory model tests. Results reveal that as the curing duration extends, the hydration rates of fly ash decrease relative to cement, being demonstrated by larger settlements at the top of fly ash-treated SDCM pile compared to that of standard SDCM pile. It is also found that elevated levels of hydration in cement mixing pile socket results in a heightened stiffness and an enhanced pile end resistance.

Numerical analysis of the local damage of RC beams under close‐in explosions based on the calibrated Karagozian & Case (K&C) concrete model

AbstractLimited research has been carried out to investigate the local damage features of reinforced concrete (RC) beams under close‐in explosions. To better capture the local damage features of rectangular cross‐sectional RC beams, a numerical study on the RC beams subjected to close‐in explosions based on the calibrated Karagozian & Case (K&C) concrete model was conducted in LSDYNA. First, a systematic calibration of the K&C model for concrete was performed in the single‐element numerical analysis. Calibration of the K&C model consists of parameter determination for the new strength surfaces, modification of the damage function to better feature the tensile behavior of concrete, elimination of the element size effect by introducing an element size factor and determination of the scaled damage factors. The new strength surfaces are verified and determined through both the approximation calculation method and the parameter fitting method. And the damage function was modified by reducing the termination of λ to better characterize the brittleness of concrete and feature the tensile failure. A comparison between the numerical results and the corresponding experimental results demonstrates that the calibrated K&C model and established FEM model of RC beams subjected to close‐in explosions can well characterize the local damage features of RC beams. On this basis, a further parameter analysis on the factors influencing the local damage response of RC beams was conducted by considering reinforcement details (stirrup spacing and longitudinal reinforcement diameter) and charge details (charge mass and aspect ratio). Results show that the close‐in explosions destroyed the concrete corners in the blast surface with shear stress first and the side shear failure of concrete corners weakens the concrete core strength and contributes to the greater loss of concrete core. Rear spalling damage was initially caused by the bond failure between the tensile longitudinal reinforcement and the surrounding concrete. Furthermore, increasing stirrup spacing, tensile longitudinal reinforcement diameter, and charge mass contribute to the increase in local damage sizes.

Behaviour of concrete-filled circular steel tubular K-joints in wind turbine towers

The advantages of concrete-filled steel tubular lattice towers in stress efficiency, material consumption, and industrial construction are significant for the future development of ultra-long blades, ultra-high towers, and large-capacity units. The stress state of the joints is a key component of a wind tower structure. Therefore, experiments were conducted on four concrete-filled circular steel tubular (CFCST) K-joints in a lattice wind turbine tower, along with one hollow circular steel tubular (HCST) K-joint for comparison. The test results revealed that the CFCST K-joints failed due to local and overall buckling of the compression web, whereas no plastic deformation occurred on the chord. The stiffness of the CFCST K-joints significantly improved, and the stress concentration around the intersecting line was significantly reduced. Existing structural codes, both domestic and international, underestimate the bearing capacity of CFCST K-joints because the contribution of the core concrete cannot be accounted for. Finite element analyses indicate that the bearing capacity of the CFCST K-joints is dependent on the failure mode, primarily influenced by the thickness ratio τ and the angle θ. To prevent joint failure in the CFCST lattice wind turbine towers, specifically suggest τ ≤ 1 and θ ≤ 45°. Based on the bearing capacity calculation method proposed by CIDECT for HCST K-joints, design equations were obtained to predict the punching shear bearing capacity of CFCST K-joints using the least-squares method. The established formula exhibited high accuracy through regression verification.

Finite Element Analysis on the Seismic Performance of Concrete-Filled Steel Tube Columns with a Multiple-Chamber Round-Ended Cross-Section

This study proposes a form of concrete-filled steel tube column with a multiple-chamber round-ended cross-section (M-CFST). Longitudinal and transverse stiffening ribs divide the circular-ended section into different chambers, strengthening the steel tube’s confinement effect on the core concrete and improving the component’s seismic performance. A three-dimensional finite element (FE) solid model of the M-CFST is created by employing the FE software ABAQUS. Quasi-static analysis is conducted to investigate the influence of parameters, such as chamber arrangement, aspect ratio, and axial compression ratio, on flexural hysteresis performance. Moreover, the failure modes, hysteresis curves, skeleton curves, strain development, and energy dissipation of the components are analyzed. The results show the following: (1) The FE model presented in this study can simulate the quasi-static behavior of CFST columns accurately, and the calculated results are in good agreement with the measured values. (2) The seismic performance of the composite column is excellent, with a large number of chambers leading to a robust hysteresis curve for the composite columns, resulting in increased bearing capacity and energy dissipation capacity. However, the energy dissipation performance of the specimen with a two-chamber arrangement is slightly lower than that with a single-chamber arrangement. (3) The results of the finite element analysis suggest that the long and short sides of the CFST columns with a large length–width ratio should be arranged to be relatively close in length.

Experimental Study on Mechanical Properties of CFRP-Confined Concrete Columns under Continuous Semi-Submergence of Sulfate

To investigate the mechanical properties of carbon-fiber-reinforced-polymer (CFRP)-restrained concrete in a saline soil environment, the degradation of the mechanical properties of CFRP-restrained concrete columns under the effect of continuous sulfate semisoak erosion is investigated based on sulfate continuous semisoak erosion, and unrestrained concrete columns with the same specifications are used in comparison tests. The results show that the strength, stiffness, and ductility of both plain concrete columns and CFRP-confined concrete columns first increase and then decrease after the continuous semi-submersion erosion by sulfate; compared with plain concrete columns, the decline rates of strength and stiffness of CFRP-confined concrete columns are significantly lower, and the CFRP demonstrates a certain protective effect on the core concrete. Through a regression analysis of experimental data, strength and ultimate strain models of CFRP-confined concrete columns under the continuous semi-submergence of sulfate are proposed based on the existing ultimate strength and ultimate strain models of CFRP-confined ordinary concrete columns, and a stress–strain model of CFRP-confined concrete columns under the continuous semi-submergence of sulfate is established. Based on a comparison with experimental data, the model prediction curves indicate good agreement with the experimental curves and can therefore provide a theoretical basis and practical reference for CFRP-reinforced semi-submerged concrete in saline soil areas.

A Concrete Core Void Imaging Approach and Parameter Analysis of Concrete-Filled Steel Tube Members Using Travel Time Tomography: Multi-Physics Simulations and Experimental Studies.

Concrete-filled steel tube (CFST) members have been widely used in civil engineering due to their advanced mechanical properties. However, internal defects such as the concrete core voids and interface debonding in CFST structures are likely to weaken their load-carrying capacity and stiffness, which affects the safety and serviceability. Visualizing the inner defects of the concrete cores in CFST members is a critical requirement and a challenging task due to the obvious difference in the material mechanical parameters of the concrete core and steel tube in CFST members. In this study, a curved ray theory-based travel time tomography (TTT) with a least square iterative linear inversion algorithm is first introduced to quantitatively identify and visualize the sizes and positions of the concrete core voids in CFST members. Secondly, a numerical investigation of the influence of different parameters on the inversion algorithm for the defect imaging of CFST members, including the effects of the model weighting matrix, weighting factor and grid size on the void's imaging quality and accuracy, is carried out. Finally, an experimental study on six CFST specimens with mimicked concrete core void defects is performed in a laboratory and the mimicked defects are visualized. The results demonstrate that TTT can identify the sizes and positions of the concrete core void defects in CFST members efficiently with the use of optimal parameters.

Spatio-temporal autogenous shrinkage and cracking behavior of core concrete in full-scale CFST: Insights from the world's largest span arch bridge

A full-scale concrete-filled steel tube (CFST) examination was performed on the Tian'e Longtan Bridge, featuring a 600-meter span and 0.9-meter steel tube diameter, to assess internal autogenous shrinkage distribution. This study employed vibrating wire strain gauges and distributed optical fibers to measure the shrinkage and detect cracking in the core concrete. Analysis of the test results led to the formulation of models that describe the axial and radial shrinkage behavior over time and space. Increased optical fiber strain highlighted regions undergoing local plastic softening and cracking. This research facilitates the early prediction and precise identification of potential cracking areas in the core concrete through optical fiber monitoring.

Experimental Study on Dynamic Characteristics of Damaged Post-Tensioning Concrete Sleepers Using Impact Hammer.

Concrete sleepers in operation are commonly damaged by various internal and external factors, such as poor materials, manufacturing defects, poor construction, environmental factors, and repeated loads and driving characteristics of trains; these factors affect the vibration response, mode shape, and natural frequency of damaged concrete sleepers. However, current standards in South Korea require only a subjective visual inspection of concrete sleepers to determine the damage degree and necessity of repair or replacement. In this study, an impact hammer test was performed on concrete sleepers installed on the operating lines of urban railroads to assess the field applicability of the modal test method, with the results indicating that the natural frequency due to concrete sleeper damage was lower than that of the undamaged state. Furthermore, the discrepancy between the simulated and measured natural frequencies of the undamaged concrete sleeper was approximately 1.87%, validating the numerical analysis result. The natural frequency of the damaged concrete sleepers was lower than that of the undamaged concrete sleeper, and cracks in both the concrete sleeper core and the rail seat had the lowest natural frequency among all the damage categories. Therefore, the damage degrees of concrete sleepers can be quantitatively estimated using measured natural-frequency values.

Operating Properties of Deep Hole Boring Tools with Modified Design.

This paper presents the results of research work on the revised design of a deep hole boring tool. The study was divided into three stages: theoretical, experimental and operational. In the theoretical part, a 3D model of the actual boring bar was created, which was subjected to strength tests using the Finite Element Method (FEM), and then prototypes of new deep hole boring tools were made with structural modifications to the shank part of the tool. For the polymer concrete core of a shank, there was a 14.59% lower displacement, and for the rubber-doped polymer concrete (SBR-styrene butadiene rubber) core of a shank there was a 4.84% lower displacement in comparison to the original boring bar. In the experimental part of the study, the original boring bar and the prototypes were subjected to experimental modal analysis and static analysis tests to compare dynamic and static properties. In the operational part of the study, boring tests were carried out for various workpiece materials, during which the basic parameters of the surface geometric structure (SGS), such as roughness Ra and Rz, were studied. Despite the promising preliminary results of the theoretical and experimental studies, using the described modifications to the boring bar is not recommended.

Interface debonding defect detection for CFST columns based on EMI measurements: Experiment, numerical simulation and blind inspection in practice

Concrete-filled steel tube (CFST) members have been widely used in skyscrapers and long-span bridges. Non-uniformly distributed hydration heat during curing, inadequate compaction during construction and unavoidable shrinkage and creep of mass concrete in service most likely lead to interface debonding defects between concrete core and steel tube, which has been a common concern. The development of effective non-destructive inspection methods for interface bonding condition of existing CFST members is critical. In this study, an interface debonding defect detection method based on electromechanical impedance (EMI) measurement using surface-mounted piezoelectric-lead-zirconate-titanate (PZT) sensors is proposed at first. Then, experimental study on the feasibility of the proposed approach is carried out with scaled CFST specimens with artificially mimicked interface debonding defects. EMI of surface-mounted PZT sensors at different frequency bands are measured and compared to verify the detectability of the proposed approach for the mimicked interface debonding defects. Thirdly, a multi-physics CFST-PZT coupling finite element model (FEM) with interface debonding defects is established, and EMI of surface-mounted PZT sensors at different locations is simulated. The influence of interface debonding defects on EMI measurements is illustrated. Finally, a blind inspection on the interface bonding condition of selected CFST columns in an existing skyscraper in service using the proposed approach is carried out. Both bonding and debonding defect in the tested CFST columns are detected correctly and validated with drilling hole observation. Experimental, numerical and engineering application results show the proposed approach is effective for interface debonding detection of CFST members and sensitive to minor debonding defect of 0.3 mm in CFST members.

Study on the flexural behaviour of the concrete filled square steel tube beam with the basic magnesium sulfate cement-based composite shell system

A new type of stay-in-place formwork (SIPF) concrete beam was proposed; this beam consists of a U-shaped shell made of basic magnesium sulfate cement-based composite (BMSCC), a perforated square steel pipe and core concrete. The prefabricated shell and square steel pipe form a composite SIPF, the steel pipe replaces the longitudinal reinforcement and hoop reinforcement, and the combination beam can be formed after pouring the core concrete inside. Four-point bending tests of two SIPF beams and one reinforced concrete (RC) comparison beam were conducted using longitudinal steel consumption as a parameter. The results showed that the SIPF beams exhibited a typical ductile damage pattern and that the precast shell was reliably bonded to the core concrete without spalling. The cracking load was increased by 198.25 %, the yield load by 66.3 %, and the ultimate load by 64.7 % for the same amount of steel used in the steel pipe instead of the reinforcing cage. After the equivalent longitudinal reinforcement rate of 1 % at the bottom of the SIPF beam was increased to 1.25 %, the flexural capacity increased by 9.65 %. By introducing the restraining stress coefficient and stiffness contribution coefficient, a formula for calculating the flexural capacity of the positive section of the SIPF beam was established.

Dynamic responses of concrete-filled steel tubes impacted horizontally by a rigid vehicle: Experimental study and numerical modelling

Concrete-filled steel tubes (CFSTs), composed of an outer tube made of steel as the confining material and an inner concrete core, have been popularly utilised as high-rise building columns and urban bridge piers owing to their superior structural behaviour. Since the invention of CFSTs, their mechanical performance under static and seismic loading conditions have been extensively studied, while the investigation on their dynamic behaviour under impact loading is relatively limited. Of these limited studies about CFSTs under impact loading conditions, most experiments were completed using a drop-hammer testing facility or a pendulum-hammer testing device. Against this background, this study investigated six large CFSTs being impacted horizontally by a rigid vehicle to analysis their dynamic response. All specimens were vertically placed with the bottom pedestal fixed on the lab floor and the top end free. The main parameters of these CFSTs included the impact velocity (i.e., vi = 3.16–7.18 m/s), the CFST section size (i.e., D = 180–300 mm), as well as the CFST column height (i.e., hc = 1500–2100 mm). Test results revealed that, (1) all CFSTs experienced a bending-dominant failure when impacted horizontally by a rigid vehicle; (2) the impact velocity had a positive correlative trend with the peak impact force Fpeak and the peak lateral displacement δpeak of the CFST column; (3) a larger section size of the CFST column resulted in a higher flexural stiffness and a larger flexural capacity, thus leading to a higher Fpeak, a smaller time duration T and a smaller δpeak; (4) the column height would affect the inertial response of the CFST column; a larger column height led to a larger Fpeak caused by the larger inertia effect of the CFST segment above the impact point. Finite-element model was established on the platform of LS-DYNA to simulate these CFSTs under horizontal impact loading, which was able to predict both impact force–time history curves and lateral displacement–time history curves with reasonably accuracy.

Experimental and numerical study on the impact response of the steel-concrete-steel-foam-filled-tube energy absorbing structure

A novel energy absorbing structure is proposed and can be fixed on the front of protected structures to reduce the impact-induced damage. This energy absorbing structure consists of nine polyurethane foam-filled steel tubes (PUFFSTs) and a steel-concrete-steel sandwich panel (SCSSP) and is named steel-concrete-steel-foam-filled-tube energy absorbing structure (SSEAS). The present study investigates the energy dissipation characteristics and dynamic responses of SSEAS through both experimental and numerical analyses. All specimens exhibited the same failure mode, including the collapse of the centre PUFFST and the local indentation of the SCSSP. The investigated parameters contain the thickness and width of the steel tube, thickness of concrete core and the initial momentum of the drop weight. A comprehensive discussion is presented on the impact response and energy dissipation performance. The results indicate that the thinner concrete core would result in larger deformation but shallower local indentation. Additionally, raising the width and thickness of the steel tube would result in higher energy absorbing capacity and larger proportion of energy dissipation of PUFFSTs. The numerical studies also show that higher initial momentum of the drop weight would induce a longer duration and larger energy dissipation of PUFFSTs.

Seismic performance of railway bridges with novel ductile piers under near-fault earthquakes

The destructive force of near-fault earthquakes presents a significant challenge to railway bridges. To enhance the seismic resilience of railway bridges under such conditions, a novelly designed ductile pier is introduced herein, accompanied by a analysis of its stiffness, damping characteristics, and shear capacity. The analysis reveals that the ductile pier exhibits reduced stiffness and increased viscous damping ratio when compared to the traditional pier. Subsequently, a finite element model of a typical railway simply-supported girder bridge is established and verified through shaking table tests. Furthermore, the near-fault seismic wave synthesis method is introduced, and the synthesized near-fault seismic waves are input into the traditional pier model and the ductile pier model for seismic analysis. The findings demonstrate that the ductile pier effectively diminishes girder acceleration and bearing displacement while enhancing pier energy dissipation. Additionally, the ductile pier is proved effective in safeguarding the core concrete during significant seismic events.

Mechanism of interface debonding in steel-concrete components caused by thermal expansion and contraction

The steel tube concrete structure constrains the core concrete through the inner wall of the steel tube, forming a hoop effect, significantly improving the compressive performance of the core concrete and effectively exerting the tensile performance of the steel tube. Although this structure has been optimized in terms of mechanical properties, its ultra-high bonding performance makes it extremely difficult to disassemble and recycle. To explore ways to solve this challenge, this article selects C40 concrete as the matrix concrete, and designs and produces circular steel tube concrete specimens and square steel tube concrete specimens with a height of 250mm and a steel tube wall thickness of 4mm. The specific research content covers the hoop effect of steel pipes on core concrete, the degradation law of thermal expansion and contraction interface bonding performance of steel pipe concrete components, the coupling effect of cutting joints and thermal expansion and contraction on interface bonding performance of steel pipe concrete components, and the mechanism of interface debonding of steel pipe concrete components. Through systematic experiments and analysis, the influence of different variation parameters on the debonding performance of steel tube concrete was deeply studied, and its debonding mechanism was revealed. This series of studies provides an important scientific basis for the theoretical understanding and application of steel-concrete structures, helping to overcome the problem of dismantling and recycling, and promoting the transformation and development of related equipment.

Static and dynamic behaviour of sandwich beams with porous core: Experiment and moving least squares mesh-free analysis

In this paper, the static and dynamic behaviour of sandwich beams with porous core are numerically analyzedand validated by experimental tests. The beam consists of a thick porous core with a uniform porosity distribution over its domain and two outer face layers. For the theoretical study, the virtual work principle is employed to derive the governing equation. A one-dimensional (1D) mesh-free approach, associated with the moving least squares Hermite interpolation, is developed to approximate the primary variable fields and discretize the governing equation. Additionally, a simple transformation method is applied to create Kronecker delta property of constructed shape functions, straightforwardly facilitating the imposition of the boundary condition, similar to the finite element method without additional techniques. The accuracy of the computational method is subsequently verified against previous literature. For the experimental tests, various mechanical responses, such as the natural frequency, static deflection, and deflection-time history of a cantilever porous sandwich beam consisting of cemboard faces and a concrete core with Expanded Polystyrene are measured and compared with the theoretical prediction. The outcomes of this study can be valuable for the design of sandwich beams with porous core.

The uncorroded-part fallacy

This paper deals with the collapse load of corroded parallel steel units, such as sets of corroded strands alone, or embedded in a concrete core. It is shown that what is relevant is the distribution of the damage between the units, and not the total area loss due to corrosion. The paper also shows that assuming that the area loss is related to the limit load loss is misleading, and potentially dangerous.